FIG. 1 illustrates a prior art image sensor 10 comprised of an array of photodiodes 20 adjacent to vertical charge couple devices (CCD) 30. The photodiodes 20 collect photo-generated charge that is then transferred to a light shielded vertical CCD 30 for read out. Such a CCD image sensor is used for capturing two images closely spaced in time (less than 100 μs). The rapid acquisition of two images is often used for determining the velocity of objects in the image. Such an image is generally referred to as particle image velocimetry (PIV). An example of a camera for PIV is the Redlake Megaplus ES 4.0 camera. A publication describing PIV with this camera is by Luiz M. Lourenco and A. Krothapalli in the Proceedings of the International Conference in Applications of Lasers to Fluid Mechanics, Lisbon, Portugal, July 2000.
The first step of acquiring a rapid sequence of two images is shown in FIG. 2. A laser or strobe flash illuminates an image scene and is collected by the photodiodes 20. The numeral 1 indicates the first image exposure charge packet in the image sensor 10. Referring to FIG. 3, the charge packets from the first image exposure are transferred to the vertical CCD 30 but not read out of the vertical CCD 30. Not reading out the first image exposure from the vertical CCD 30 saves time and allows a second laser pulse to illuminate the second image capture in the photodiodes as illustrated in FIG. 4. The time period between the first and second laser pulses can be as short as 5 μs. After the second laser pulse, the second image is held in the photodiodes 20 until the first image is read out of the vertical CCD 30. Now referring to FIG. 5, when the vertical CCD 30 is empty the second image is transferred into the vertical CCD 30. Referring to FIG. 6, finally the second image is read out of the vertical CCD 30.
One disadvantage to the image capture sequence outlined in FIGS. 2 through 6 is only two images can be captured in rapid sequence. Consequently, a need exists for capturing more than two images in sequence. The present invention will disclose how to capture three images in rapid sequence.